The development of solid state light sources, such as but not limited to light emitting diodes (LEDs), has led to use of such devices in various lighting applications and fixtures. In general, an LED is a current driven device, i.e. the brightness of the light output of an LED may be directly related to the current supplied to the LED. However, the current through an LED should be limited to a design maximum to avoid over-current damage or failure of the LED. In general, single LED light sources operate at only 2 to 4 volts and therefore require a low voltage power supply to power them. Furthermore, the current through the LED is strongly dependent on the applied voltage and also on the temperature of the LED. For this reason, LED light sources are typically powered by a ballast and/or driver that transforms the mainline voltage to a level useable by the LED light source, and also controls the current. An additional function of the ballast and/or driver is to condition the current drawn from the mains supply to present the mains with a high power factor load having low harmonic distortion.
In general, it is desirable to operate multiple LEDs in order to achieve a higher light level. In this case, strings of LEDs are connected in series and powered by a ballast and/or a driver that supply the voltage needed and maintain the desired current level. Such a ballast and/or driver is typically a complex piece of electronic equipment containing numerous inductors, power transistors, large electrolytic capacitors, and integrated circuits. Some of these components, particularly the capacitors, are not tolerant of elevated temperatures. Thus, the ballast and/or driver is often a life-limiting piece of the system. Additionally, such circuitry is typically expensive to produce. At the other extreme, a simple series resistor is an inexpensive way to power a series string of LEDs, but results in high harmonic distortion as the current waveform is not sinusoidal. Additionally, it is an inefficient circuit as the resistor dissipates power.
Unipolar driving of solid state light sources connected in series (e.g., a string of LEDs) is described in, for example, U.S. application Ser. No. 13/229,611 (by the same inventors of the current application). Using such a unipolar driver, the average intensity of the solid state light sources is not uniform along the string owing to differences in the power applied to each source as instantaneous string voltage changes. This requires special physical placement of the LEDs in any practical system so this intensity variation is masked.